Methods of operating a wind turbine

ABSTRACT

Methods of operating a variable speed wind turbine as a function of a wind speed, the wind turbine having a rotor with a plurality of blades, and one or more pitch mechanisms for rotating the blades. The method comprising a sub-nominal zone of operation for wind speeds below the nominal wind speed and a supra-nominal zone of operation for wind speeds above the nominal wind speed. In the supra-nominal zone, the blades are pitched so as to maintain the rotor speed substantially constant, and a tip speed ratio of the wind turbine is substantially continuously being determined and wherein an instantaneous minimum pitch angle is substantially continuously being determined based on the instantaneous tip speed ratio, and the blades are never pitched below the instantaneous minimum pitch angle. The disclosure further relates to a wind turbine suitable for carrying out such methods.

This application claims the benefit of European Patent ApplicationEP13382013.4 filed on Jan. 17, 2013 and U.S. Provisional PatentApplication Ser. No 61/802,967 filed on Mar. 18, 2013.

The present disclosure relates to methods of operating a wind turbine.

BACKGROUND ART

Modern wind turbines are commonly used to supply electricity into theelectrical grid. Wind turbines of this kind generally comprise a rotorwith a rotor hub and a plurality of blades. The rotor is set intorotation under the influence of the wind on the blades. The rotation ofthe rotor shaft either directly drives the generator rotor (“directlydriven”) or through the use of a gearbox.

A variable speed wind turbine may typically be controlled by varying thegenerator torque and the pitch angle of the blades. As a result,aerodynamic torque, rotor speed and electrical power will vary.

A common prior art control strategy of a variable speed wind turbine isdescribed with reference to FIG. 1. In FIG. 1, the operation of atypical variable speed wind turbine is illustrated in terms of the pitchangle (β), the electrical power generated (P), the generator torque (M)and the rotational velocity of the rotor (ω), as a function of the windspeed.

In a first operational range, from the cut-in wind speed to a first windspeed (e.g. approximately 5 or 6 m/s), the rotor may be controlled torotate at a substantially constant speed that is just high enough to beable to accurately control it. The cut-in wind speed may be e.g.approximately 3 m/s.

In a second operational range, from the first wind speed (e.g.approximately 5 or 6 m/s) to a second wind speed (e.g. approximately 8.5m/s), the objective is generally to maximize power output whilemaintaining the pitch angle of the blades constant so as to capturemaximum energy. In order to achieve this objective, the generator torqueand rotor speed may be varied so as keep the tip speed ratio λ(tangential velocity of the tip of the rotor blades divided by theprevailing wind speed) constant so as to maximize the power coefficientC_(p).

In order to maximize power output and keep C_(p) constant at its maximumvalue, the rotor torque may be set in accordance with the followingequation:T=k·ω²,whereink is a constant, and ω is the rotational speed of the generator. In adirect drive wind turbine, the generator speed substantially equals therotor speed. In a wind turbine comprising a gearbox, normally, asubstantially constant ratio exists between the rotor speed and thegenerator speed.

In a third operational range, which starts at reaching nominal rotorrotational speed and extends until reaching nominal power, the rotorspeed may be kept constant, and the generator torque may be varied tosuch effect. In terms of wind speeds, this third operational rangeextends substantially from the second wind speed to the nominal windspeed e.g. from approximately 8.5 m/s to approximately 11 m/s.

In a fourth operational range, which may extend from the nominal windspeed to the cut-out wind speed (for example from approximately 11 m/sto 25 m/s), the blades may be rotated (“pitched”) to maintain theaerodynamic torque delivered by the rotor substantially constant. Inpractice, the pitch may be actuated such as to maintain the rotor speedsubstantially constant. At the cut-out wind speed, the wind turbine'soperation is interrupted.

In the first, second and third operational ranges, i.e. at wind speedsbelow the nominal wind speed (the sub-nominal zone of operation), theblades are normally kept in a constant pitch position, namely the “belowrated pitch position”. Said default pitch position may generally beclose to a 0° pitch angle. The exact pitch angle in “below rated”conditions however depends on the complete design of the wind turbine.

The before described operation may be translated into a so-called powercurve, such as the one shown in FIG. 1. Such a power curve may reflectthe optimum operation of the wind turbine under steady-state conditions.However, in non-steady state (transient) conditions, the operation maynot necessarily be optimum.

As further background, basic aerodynamic behaviour of (the blades of) awind turbine is explained with reference to FIGS. 2a -2 f.

In FIG. 2a , a profile of a wind turbine blade is depicted in operation.The forces generated by the aerodynamic profile are determined by thewind that the profile “experiences”, the effective wind speed V_(e). Theeffective wind speed is composed of the axial free stream wind speedV_(a) and the tangential speed of the profile V_(t). The tangentialspeed of the profile V_(t) is determined by the instantaneous rotorspeed ω and the distance to the centre of rotation of the profile, thelocal radius r, i.e. V_(t)=ω·r.

The axial free stream wind speed V_(a) is directly dependent on the windspeed V_(w), and on the speed of the wind downstream from the rotorV_(down), that is V_(a)=½(V_(w)+V_(down)). The axial free stream windspeed may e.g. be equal to approximately two thirds of the wind speedV_(w).

The resultant wind flow, or effective wind speed V_(e), generates lift Land drag D on the blade. A blade may theoretically be divided in aninfinite number of blade sections, each blade section having its ownlocal radius and its own local aerodynamic profile. For any given rotorspeed, the tangential speed of each blade section will depend on itsdistance to the rotational axis of the hub (herein referred to as localradius).

The lift generated by a blade (section) depends on the effective windspeed V_(e), and on the angle of attack of the blade (section) α, inaccordance with the following formula:L=½ρ·C _(L) V _(e) ² ·S,whereinρ is the air density, V_(e) is the effective wind speed, C_(L) is thelift coefficient (wherein the lift coefficient is dependent on the angleof attack α) and S is the surface of the blade section.

Similarly, the drag D generated by a blade section can be determined inaccordance with the following equation:D=½ρ·C _(D) V _(e) ² ·S,wherein C _(D) is the drag coefficient dependent on angle of attack α.

For an entire wind turbine blade, the contribution to lift and drag ofeach blade section may be summed to arrive at the total drag and liftgenerated by the blade.

Both the drag coefficient C_(D) and the lift coefficient C_(L) depend onthe profile or the blade section and vary as a function of the angle ofattack of the blade section. The angle of attack α may be defined as theangle between the chord line of a profile (or blade section) and thevector of the effective wind flow, see also FIG. 2 a.

FIG. 2b illustrates in a very general manner how the lift coefficientand drag coefficient may vary as a function of the angle of attack of ablade section. Generally, the lift coefficient (reference sign 21)increases to a certain maximum at a so-called critical angle of attack23. This critical angle of attack is also sometimes referred to as stallangle. The drag coefficient (reference sign 22) may generally be quitelow and starts increasing in an important manner close to the criticalangle of attack 23. This rapid change in aerodynamic behaviour of aprofile or blade section is linked generally to the phenomenon that theaerodynamic flow around the profile (or blade section) is not able tofollow the aerodynamic contour and the flow separates from the profile.The separation causes a wake of turbulent flow, which reduces the liftof a profile and increases the drag significantly.

The exact curves of the lift coefficient and drag coefficient may varysignificantly in accordance with the aerodynamic profile chosen.However, in general, regardless of the aerodynamic profile chosen, atrend to increasing lift up until a critical angle of attack and also arapid increase in drag after a critical angle of attack can be found.

In accordance with FIG. 2a , the tangential force generated by a bladesection is given by T=L·sin(α+ϑ)−D·cos(α+ϑ), wherein ϑ is the pitchangle and α is the angle of attack. The pitch angle may be defined asthe angle between the rotor plane and the chord line of a profile.Integrating the tangential force distribution over the radius providesthe driving torque.

In order to increase the torque generated by the rotor, the angle ofattack of any blade section is preferably kept below the critical angleof attack such that lift may be higher and drag may be lower.

It should be borne in mind that the angle of attack of each bladesection depends on the tangential speed of the specific rotor bladesection, the wind speed, the pitch angle and the local twist angle ofthe blade section. The local twist angle of a blade section maygenerally be considered constant, unless some kind of deformable bladeis used. The tangential speed of the rotor blade section depends on therotor speed (angular velocity of the rotor which is obviously the samefor the whole blade and thus for each blade section) and on the distanceof the blade section to the rotational axis.

For a given pitch angle, it follows that the angle of attack isdetermined by the tip speed ratio:

$\lambda = {\frac{\omega.R}{V_{w}}.}$From this, it follows that the torque generated by a rotor blade sectionmay become a rather complicated function of the instantaneous tip speedratio and the pitch angle of the blade. This complicated relationshipmay be illustrated with a figure such as FIG. 2 c.

For every rotor blade section, the torque generated may be correlated toone of the lines of FIG. 2d of constant pitch angle. These lines depictthe power coefficient (C_(p)), i.e. the ratio between the mechanicalpower captured by the wind turbine rotor and the available power in thewind, as a function of tip speed ratio λ and for different pitch angles.They may be obtained as cross-sections of planes of constant pitch anglewith a figure such as the one shown in FIG. 2 c.

As the power captured by the wind turbine is directly related to thegenerated torque, C_(p) curves provide information about the torquedependence on pitch angle. For each pitch angle, there is a certaincritical tip speed ratio. Below this tip speed ratio, stall may occur,i.e. the angle of attack is higher than the previously mentionedcritical angle of attack.

This may be illustrated in an alternative manner, such as shown in FIG.2e . For a given tip speed ratio, e.g. λ₁, there is a certain criticalpitch angle ϑ_(crit). This critical pitch angle gives thebefore-mentioned critical angle of attack for the given tip speed ratio.Below that critical pitch angle, stall may occur. At the same time forthe given tip speed ratio, at the critical pitch angle, the capturedpower is maximum.

Now again with reference to FIG. 1, if the wind turbine is operating inthe fourth operational range, i.e. in the supra-nominal zone ofoperation, the blades are pitched in an attempt to maintain the torqueconstant as the wind speed changes. In examples, this operational rangemay extend from the nominal wind speed to the cut-out wind speed.

It would be very hard to accurately adjust the pitch angle in responseto wind speed measurements as obtained from a nacelle anemometer. Anacelle mounted anemometer will generally, due its location on top ofthe nacelle and behind the rotor, not measure the wind speed veryaccurately and its measurements may show a wind speed that largelyvaries with a high frequency. If the pitch system were to actuate onthese measurements, it would constantly adjust the blade pitch (whichwould lead to premature wear of the pitch system) and the pitch systemwould not even be able to follow the commands that vary constantly. Andif one also takes into account effects such as wind shear and wind veer,which cannot even be registered with a nacelle mounted anemometer, itbecomes clear that the anemometer cannot be used for deriving pitchsignals.

In practice therefore, instead of using measurements from an anemometer,the rotor speed is used. The rotor speed may be measured e.g. bymeasuring the generator rotor speed. In direct drive wind turbines, therotor speed will correspond to the generator rotor speed, and in windturbines employing a gearbox, there will generally be a fixed ratiobetween generator rotor speed and rotor speed.

The pitch system is then actuated in such a manner as to keep the rotorspeed constant. This may work well in steady-state conditions or almoststeady-state conditions, but in conditions which change relativelyquickly, this may lead to undesirable results. This may be illustratedfurther with reference to FIG. 2 e.

Suppose a situation in which the wind turbine is operating in thesupra-nominal zone with a tip speed ratio λ₁. In this zone, the pitchangle will generally not be close to the critical pitch angle, whereinthe critical pitch angle if the pitch angle corresponding for thisparticular situation to a critical angle of attack. The pitch angle willbe higher than the critical angle, as the pitch of the blades is used toreduce the loads on the rotor and maintain aerodynamic torquesubstantially constant.

Let's now suppose that a sudden wind gust occurs, i.e. a significantincrease in wind speed in a relatively short time. Due to the inertia ofthe rotor, the rotor speed will not immediately increase. As aconsequence, also the pitch system will not immediately react to theincrease in wind speed. It may be however, that due to the increase inwind speed, the wind turbine is now operating at another tip speedratio, e.g. λ₂. (because the wind speed changes, but the rotor speed hasnot changed).

At this other tip speed ratio, λ₂, with the same pitch angle, stall mayoccur in the wind turbine blades, since the angle of attack of theblades may be above the critical angle of attack. With reference to FIG.2e , the point of operation may have moved from point A to point B.

That is, in order to maintain an optimum angle of attack at the momentof the sudden wind gust, the pitch angle should be increased.Nevertheless, as the pitch system depends on the rotor inertia, itcannot track a sudden wind change, so blade pitch remains somewhatstuck, thus resulting in a large angle of attack. Depending on theprecise effects of the wind, and the inertia of the rotor, it may bethat the rotor speed even decreases a little bit, due to the separationof the flow from the blades. In response to this decrease in rotorspeed, the pitch system will reduce the pitch angle more, thusaggravating the situation by further increasing the angle of attack.

The above situation may be particularly troublesome in case of e.g. aMexican hat wind gust, such as the ones depicted in FIG. 2f . Mexicanhat wind gusts are defined in the IEC 64100-1 2nd edition 1999-02standard, since they may be particularly dangerous wind gusts. Thisstandard defines Mexican hat wind gusts at various speeds, and atvarious azimuth angles.

The loads a wind turbine suffers during such a wind gust are severe andmay define design loads for the wind turbine. This is due to thedecrease in wind speed, before the high increase in wind speed (see FIG.1). When the wind speed decreases, the pitch system tries to adapt theblades to this decrease (the blades are initially rotated in such a wayto increase the aerodynamic torque by increasing their angle of attack,i.e. the pitch rate is below zero). With the pitch adaptation stillongoing, a significant increase in wind speed occurs. The aerodynamictorque and the thrust force on the hub can thus be very high. The pitchof the wind turbine will then start to adapt to these new windconditions. However, the wind speed keeps increasing and due to theinertia of the system, the pitch can possibly not be adapted quicklyenough, thus leading to the wind turbine potentially stalling andsuffering increased loads. A typical pitch system may have an inherentpitch limitation of approximately 5°/second. Such a pitch rate may inprinciple be fast enough to respond to wind variations occurring duringoperation of the wind turbine. In general, the limiting factor may notbe the pitch drive system but the means used to sense wind speed, i.e.rotor speed.

There still exists a need for a method of operating a wind turbine thatat least partially reduces the aforementioned problems.

SUMMARY

In a first aspect, a method of operating a variable speed wind turbineas a function of a wind speed is provided. The wind turbine has a rotorwith a plurality of blades, and one or more pitch mechanisms forrotating the blades.

The method comprises a sub-nominal zone of operation for wind speedsbelow the nominal wind speed and a supra-nominal zone of operation forwind speeds above the nominal wind speed. According to this aspect, inthe supra-nominal zone, the blades are pitched so as to maintain therotor speed substantially constant, and a tip speed ratio of the windturbine is substantially continuously being determined and wherein aninstantaneous minimum pitch angle is substantially continuously beingdetermined based on the instantaneous tip speed ratio, and the bladesare never pitched below the minimum pitch angle.

In accordance with this aspect, the wind turbine can follow normaloperation for (substantially) steady-state conditions. At the same time,the problem of stalling of the blades in case of transients can beavoided; the tip speed ratio is monitored substantially continuously(i.e. with a frequency high enough to adapt for wind changes). Thefrequency of determining the tip speed ratio may e.g. be every 0.5seconds, every second, or every few seconds. By determining the tipspeed ratio “in real-time”, at any time, the minimum pitch angle thatshould not be surpassed is known. Using this information, this minimumpitch angle is set as a boundary condition for normal operation, or atleast in the supra-nominal zone of operation. In the supra-nominal zoneof operation, the actual pitch angle will normally be significantlyhigher than this minimum pitch angle and normal operation will thus notbe affected. Only in case of transients, the added boundary conditionhelps the wind turbine to operate better.

This improved operation may be achieved without the need for adding anyspecial equipment or sensors. In conventional wind turbines, a nacellemounted anemometer is generally present. The generator or rotor speed isnormally measured in conventional wind turbines as well.

In some embodiments, the minimum pitch angle is defined as the pitchangle corresponding to a critical angle of attack of a wind turbineblade section. The critical angle of attack of a wind turbine bladesection is the angle of attack at which the wind turbine blade sectionstarts to stall. Importantly, the precise angle of attack for anysection of a blade may be different, as it depends on local twist angleand the distance of the section to the rotational axis, as well as onthe wind speed and pitch angle, the last two factors being common forthe whole blade, but the first two varying along the blade.

In alternative embodiments, the minimum pitch angle may be defined asthe pitch angle corresponding to an angle of attack that is a predefinedamount or percentage below a critical angle of attack of a wind turbineblade section. This predefined amount or percentage may be regarded as asecurity measure.

In some examples, the representative wind turbine blade section may bechosen as the section at 25% of the blade length.

In a second aspect, a wind turbine is provided having a generator, arotor with a plurality of blades, one or more pitch mechanisms forrotating the blades, a system for determining the rotor speed, a windspeed sensor, and a control system comprising a pitch control system anda generator control system. The generator control system is adapted tocontrol the generator and the pitch control system is adapted to controlthe pitch mechanisms. The generator control system is adapted todetermine generator torque commands in accordance with a steady-statecontrol loop and the pitch control system is adapted to determine pitchcommands in accordance with a transient control loop and thesteady-state control loop. The steady-state control loop is adapted tocalculate generator torque set commands and pitch set commands based atleast partially on the instantaneous rotor speed determined by thesystem for determining the rotor speed. The transient control loop isadapted to calculate an instantaneous tip speed ratio based on a windspeed measured by the wind speed sensor and the instantaneous rotorspeed determined by the system for determining the rotor speed, and isfurther adapted to determine instantaneous minimum pitch commands basedon the instantaneous tip speed ratio. In a supra-nominal zone ofoperation for wind speeds above a nominal wind speed, the pitch controlsystem is adapted to determine whether the pitch set command is abovethe instantaneous minimum pitch command, and in case of positive result,follow the received pitch set command. In case of negative result, thepitch control system is adapted to follow the instantaneous minimumpitch command.

In this aspect, a wind turbine is provided with a control system thatmay be adapted to follow a predefined power curve. In a supra-nominalzone of operation, the wind turbine may thus be adapted to varying windspeeds by adjusting a pitch angle of the blades in order to maintainrotor speed (or generator speed) constant. At the same time, at least inthis supra-nominal zone, an instantaneous minimum pitch angle ismonitored constantly and the control system ensures that this minimumpitch angle is superposed as a minimum boundary on the normal pitchcontrol. Aerodynamic stall of the rotor blades may thus be avoided.

Additional objects, advantages and features of embodiments of theinvention will become apparent to those skilled in the art uponexamination of the description, or may be learned by practice of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments of the present invention will be described in thefollowing by way of non-limiting examples, with reference to theappended drawings, in which:

FIG. 1 illustrates a typical power curve of a wind turbine;

FIGS. 2a-2f illustrate aerodynamics of wind turbine blades andaerodynamic profiles in general;

FIG. 3 illustrates a wind turbine;

FIGS. 4a and 4b illustrate aerodynamics related to examples of methodsaccording to the present invention; and

FIG. 5 illustrates a wind turbine and a control system according to anexample of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a wind turbine 40 having a rotor with three blades41, 42 and 43. A nacelle 45 is mounted on wind turbine tower 44. Ananemometer 46 is mounted on the nacelle 45. The anemometer 46 may beused to measure wind speed, however because of its location on thenacelle, behind the rotor, the wind speed measured by the anemometer mayvary a lot and in general may not be very reliable.

The wind turbine may have one or more pitch systems to rotate the blades41, 42, 43 collectively or individually. FIG. 1 represents a typicalpower curve for a variable speed wind turbine with pitch capability. Asmentioned before, above the nominal wind speed, the generator torque androtor speed may be maintained constant even though the wind speedincreases. This can be achieved by pitching the wind turbine blades,i.e. by rotating the blades along their longitudinal axes and withrespect to the hub; by increasing the pitch of the blades, their angleof attack decreases and their contribution to the torque also decreases.

The ideal power curve as depicted in FIG. 1 may most reliably befollowed during steady-state conditions if the pitch system(s) is/aredirectly controlled by a sensor indicating the rotor speed. Therotational speed of the rotor may be measured directly or may bedetermined by measuring the rotational speed of the generator rotor. Thepitch system is thus actuated to keep the speed constant, and ifgenerator torque is maintained constant as well, this means thataerodynamic torque is maintained substantially constant as well.

A problem arises in case of transients wind conditions. A particularlychallenging situation may be a Mexican hat wind gust in thesupra-nominal zone of operation. However, examples of the presentinvention may equally well be suitable in different transientconditions, such as sudden increases or decreases of wind speeds, forexample in case of increased turbulence.

In the case of a Mexican hat wind gust, the wind speed first decreases.Although the rotor may be slow to react, the rotor speed may diminish alittle bit. The reaction of the pitch system may thus be to increase theangle of attack of the blades (i.e. decrease the pitch) so that theblades capture the wind “better”, i.e. increase their lift.

After the decrease in wind speed, the wind speed suddenly increases.Once again, the rotor will be slow to react. The rotor speed mayincrease, and in reaction the pitch of the blades may be increased, butthis may not be sufficient. Due to the previous decrease in pitch andcorresponding increase in angle of attack, when the wind speed starts torise, the angle of attack of the blade may surpass the critical angle ofattack, and the blades may stall. With reference to FIG. 4a , the pointof operation moves from point A, at tip speed ratio equal to λ₁, topoint B, which results from a similar pitch (the inertia of the rotor isnot capable of tracking wind variations, so the input to the pitch drivesystem remains almost unaltered during the wind gust) and asignificantly reduced tip speed ratio, λ₂.

A consequence of the stall may be relatively high loads. Anotherconsequence may be a decrease in rotor speed. As such, the reaction ofthe pitch system may be to further decrease the pitch (and increase theangle of attack), thus aggravating the stall.

In accordance with some examples of the invention, the tip speed ratiomay be determined in a substantially continuous manner by measuring bothwind speed and (generator) rotor speed. Given the real-time tip speedratio, a minimum pitch angle may be determined which corresponds to amaximum angle of attack of the blade which should not be surpassed. Thismaximum angle of attack of the blade may correspond substantially to acritical angle of attack. Alternatively, it may correspond to an angleof attack with a predefined distance to the critical angle of attack.

In the aforementioned example, when the wind speed starts to increase,the anemometer will be able to measure the increase in wind speed beforethe rotor speed increases. Whereas the input for the pitch systemnormally is the rotor speed, in this case, a boundary condition is setfor the pitch angle not to surpass a minimum pitch angle as determinedbased on the anemometer (or LIDAR or other wind measuring device). Withreference to FIG. 4a , the point of operation moves from point A topoint C (instead of to point B), which is located to the right ofϑ_(crit,2), that is, to the right of the pitch angle that would lead tostalling of the blade. The blades will thus not stall and operation maycontinue in a more efficient manner, with lower blade loads.Furthermore, tower loads are also reduced as situations with high windand low speed are avoided. These situations result in high thrust on thewind turbine rotor which would lead to abnormally high loads on thetower as well.

The same effect can be also explained with reference to FIG. 4b . Thecurve indicated with “normal” illustrates the blade pitch angle as afunction of the tip speed ratio under steady-state conditions as may bedetermined in accordance with a power curve such as the one illustratedin FIG. 1.

The curve indicated with the label “critical” illustrates that for eachtip speed ratio, a critical pitch angle, ϑ_(crit), exists. This curvedefines the blade pitch lower boundary. Thus, given a certain tip speedratio, pitch values below the second curve may result in stall. Thesepitch values are therefore to be avoided in order to keep loads undercontrol.

As already mentioned with reference to FIG. 4a , an initial steady statesituation might correspond to the wind turbine operating in thesupra-nominal zone with a tip speed ratio λ₁. Under these circumstances,a blade pitch angle on the steady-state curve, ϑ₁, would normally befound (operating point shown as “A”).

In case of a wind gust, a significant shift may result in the tip speedratio, which may be reduced from λ₁ to λ₂ Nevertheless, as the rotorinertia is too large to track said wind variations, the pitch set pointas defined by prior-art methods, remains almost unaffected around ϑ₁(operating point “B”). Consequently, the new operating point, which ischaracterized by a too low pitch angle for the prevailing tip speedratio, lies in the unstable region. Stall may occur, and loads mayincrease.

This problem may be avoided, according to the present invention, byusing a second blade pitch setpoint that defines a minimum boundary.This value is not dependent on the rotor speed but on the tip speedratio. In one example, the curve labelled “critical”, defining ϑ_(crit)values for each tip speed ratio may be used as this minimum boundary.The resulting operational point would thus be point C₂.

In another example, a curve defining minimum pitch angle values whichlie slightly above the critical pitch angles may be used as such aminimum boundary. In FIG. 4b , such a curve is labelled with “minimum”.The resulting operating point may thus be point C₁.

The minimum pitch values may in some embodiments be e.g. the pitch anglecorresponding to an angle of attack that is a predefined amount orpercentage below a critical angle of attack of a representative windturbine blade section for the supra-nominal zone of operation.

Even though in FIG. 4b the curves giving the relationship between tipspeed ratio and pitch angle may be straight lines, it is to beunderstood that this is not necessarily the case.

FIG. 5 illustrates a wind turbine and a control system according to anexample of the present invention. Wind turbine 40 comprises a generatorand one or more pitch actuators 65. The pitch actuators or pitchmechanisms may be controlled by a pitch control system 60. To this end,the pitch control system 60 may send pitch commands 61 to the pitchactuators 65. A result is the setting of pitch angle 66 in the windturbine 40.

A generator control system 50 may send torque commands to a generator 55and a converter 53 related to the generator. A result is the setting ofthe generator torque 56 in the wind turbine 40.

Results of both settings in the wind turbine include a generator speed,ω_(gen), and electrical power P generated.

A control system of the wind turbine may comprise a steady state controlloop 70 and a transient control loop 80. The steady state control loop70 may be adapted generally to control the wind turbine in such a waythat a predefined power curve, e.g. such as the one in FIG. 1 isfollowed. The transient control loop may be adapted to ensure that theinstantaneous pitch angles of the blades do not sink below a minimumpitch angle. This minimum pitch angle may be e.g. a critical pitch anglefor a representative portion of the blade, e.g. at 25% of the bladelength.

The steady state control loop 70, a generator speed sensor 36 maymeasure the generator speed, ω_(meas). A comparison of the measuredgenerator speed, ω_(meas), with an expected generator speed ω_(steady)gives an error result ε. Based on the error, the torque controller 50and the pitch controller 60 can determine pitch commands 61 andgenerator torque commands 51. Depending on the instantaneous operationalrange, the generator torque or the pitch angle of the blades, or bothmay be adapted to generally follow the predefined power curve.

The transient control loop 80, comprises a tip speed ratio calculatorTSR. The calculation of an instantaneous tip speed ratio may be based ona wind speed V_(wind) as measured by a nacelle mounted anemometer 46.The measurement from the anemometer upon which the determination of theminimum pitch angle is based, may be an average wind speed as measuredby the anemometer over a period of e.g. 1-5 seconds, e.g. 3 seconds.Alternatively, any other system for determining a representative windspeed can be used, such as e.g. a LIDAR.

The calculation of the instantaneous tip speed ratio may be based on themeasured generator speed, ω_(meas). The rotor speed used in thecalculation may be the measured generator speed in the case of directdrive wind turbines, or may have a constant ratio with the generatorspeed in the case of wind turbines with a gearbox.

The calculated instantaneous tip speed ratio, λ, may be sent to aminimum pitch commander 69. Based e.g. on curves such as the ones shownin FIG. 4b , the minimum pitch commander may send instantaneous minimumpitch commands ϑ_(min) to the pitch controller 60. The pitch command 61sent to the pitch mechanism(s) may be the pitch set command determinedin the steady state control loop if it is above the minimum pitchcommand. If the pitch set command of the steady state control loop isbelow the instantaneous minimum pitch command the instantaneous minimumpitch command (as determined in the steady state control loop) isfollowed and sent to the pitch mechanism(s).

In a supra-nominal zone of operation, corresponding to wind speeds abovenominal wind speeds, the steady state control loop may send constantgenerator torque commands to the converter 53 and generator 55 andvarying pitch commands 61 to the pitch actuator(s) 65. The pitchcommands are varied so as to maintain the generator speed constant. Atthe same time, the transient control loop 80 ensures that the pitchangle does not fall below a minimum pitch angle, so that the blade doesnot stall.

Although only a number of particular embodiments and examples of theinvention have been disclosed herein, it will be understood by thoseskilled in the art that other alternative embodiments and/or uses of theinvention and obvious modifications and equivalents thereof arepossible. Furthermore, the present invention covers all possiblecombinations of the particular embodiments described. Thus, the scope ofthe present invention should not be limited by particular embodiments,but should be determined only by a fair reading of the claims thatfollow.

The invention claimed is:
 1. A method of operating a variable speed wind turbine as a function of a wind speed, the wind turbine baying a rotor with a plurality of blades, one or more pitch mechanisms for rotating the blades, a sub-nominal zone of operation for wind speeds below a nominal wind speed and a supra-nominal zone of operation for wind speeds above the nominal wind speed, wherein when the wind turbine is operating in the supra-nominal zone the method comprises: an instantaneous tip speed ratio of the wind turbine is substantially continuously determined, an instantaneous minimum pitch angle is substantially continuously determined based on the instantaneous tip speed ratio, and the blades are pitched so as to maintain a substantially constant rotor speed, except that in the case of transient wind conditions the blades are never pitched below the instantaneous minimum pitch angle, wherein the minimum pitch angle is defined as the pitch angle corresponding to an angle of attack that is a predefined amount or percentage below a critical angle of attack of a representative wind turbine blade section.
 2. The method according to claim 1, wherein the minimum pitch angle is defined as the pitch angle corresponding to a critical angle of attack of a representative wind turbine blade section.
 3. The method according to claim 1, wherein the minimum pitch angle is defined as the pitch angle corresponding to an angle of attack that is a predefined amount or percentage below a critical angle of attack of a representative wind turbine blade section.
 4. The method according to claim 3, wherein the critical angle of attack is defined as the critical angle of attack for the blade section at approximately 25% of the blade length.
 5. The method according to claim 1, wherein the tip speed ratio is determined based on a wind speed measurement of a nacelle mounted anemometer.
 6. The method according to claim 5, wherein the wind speed measurement of the nacelle mounted anemometer is an average wind speed measured over a short period of time.
 7. The method according to claim 6, wherein the short period of time is 1-5 seconds.
 8. The method according to claim 7, wherein the short period of time is between 2-4 seconds.
 9. The method according to claim 8, wherein the short period of time is approximately 3 seconds.
 10. The method according claim 1, wherein the tip speed ratio is determined based on a generator speed. 